Compositional and structural gradients for fuel cell electrode materials

ABSTRACT

A fuel cell includes at least one electrode operatively disposed in the fuel cell, and having a catalytically active surface. The present invention further includes a mechanism for maintaining a substantially uniform maximum catalytic activity over the surface of the electrode.

BACKGROUND OF THE INVENTION

The present invention relates generally to fuel cells, and moreparticularly to fuel cells having electrodes with compositional and/orstructural gradients.

Fuel cells use an electrochemical energy conversion of a fuel andoxidant into electricity and heat. It is anticipated that fuel cells maybe able to replace primary and secondary batteries as a portable powersupply. In fuel cells, the fuel (containing a source of hydrogen orother oxidizable compound) is oxidized with a source of oxygen toproduce (primarily) water and carbon dioxide. The oxidation reaction atthe anode, which liberates electrons, in combination with the reductionreaction at the cathode, which consumes electrons, results in a usefulelectrical voltage and current through the load.

As such, fuel cells provide a direct current (DC) voltage that may beused to power motors, lights, electrical appliances, etc. A solid oxidefuel cell (SOFC) is one type of fuel cell that may be useful in portableapplications, as well as in many other applications.

A significant amount of effort has been expended in optimizingcomposition and porosity of electrodes. Typical approaches have involvedelectrodes formed from materials having a constant compositional andstructural morphology. More recently, a structural and/or compositionalgradient of the electrode in the direction away from the electrolyteappears to provide some benefit in improving performance of SOFCsystems. Unfortunately, in both cases, compromises are necessarily maderelating to operating temperatures, fuel cell performance, and fuelutilization when using materials with such morphologies.

SUMMARY OF THE INVENTION

The present invention solves the drawbacks enumerated above by providinga fuel cell including at least one electrode operatively disposed in thefuel cell, and having a catalytically active surface. The presentinvention further includes a mechanism for maintaining a substantiallyuniform maximum catalytic activity over the surface of the electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

Objects, features and advantages of embodiments of the present inventionmay become apparent upon reference to the following detailed descriptionand drawings, in which:

FIG. 1 is a semi-schematic cross-sectional perspective view of anembodiment of the present invention, showing fuel cell assemblies andthe gas flow passage in an embodiment of a single chamber fuel cell;

FIG. 1A is a schematic view an alternate embodiment of a single chamberfuel cell;

FIG. 1B is a schematic view of another alternate embodiment of a singlechamber fuel cell;

FIG. 1C is a schematic view of an embodiment of a dual chamber fuelcell, showing in phantom optional inlet(s) downstream for fuel and/orair;

FIG. 2 is a block diagram of an embodiment of an anode fuel/air mixturegradient for a single chamber fuel cell, schematically showing amanifold and the flow passage in phantom;

FIG. 3 is a block diagram showing an embodiment of an anodecompositional gradient;

FIG. 4 is a block diagram showing a further embodiment of an anodecompositional gradient;

FIG. 5 is a block diagram showing an embodiment of an anode structuralgradient;

FIG. 6 is a block diagram of an embodiment of a cathode compositionalgradient;

FIG. 7 is a block diagram of an embodiment of a cathode structuralgradient;

FIG. 8A is a block diagram of an embodiment of an anode portion of afuel cell stack;

FIG. 8B is a block diagram of an embodiment of a cathode portion of afuel cell stack; and

FIG. 9 is a schematic representation of an embodiment of a method formaking an anode having compositional gradient(s).

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention is predicated upon the unexpected and fortuitousdiscovery that performance of a fuel cell may be improved by varying thecomposition and/or structure of fuel cell electrodes (anodes/cathodes)with respect to the distance from the gas inlet to maximize thecatalytic activity to specific reactions related to the composition ofthe gas along the flow path, and/or by varying the composition of thegas (fuel and/or oxidant) over the catalytically active surface of anelectrode (with or without compositional and/or structural gradients).

It is to be understood that, throughout this disclosure, the definitionof “structure” and/or “structural” is intended to include morphology,porosity, crystalline structure, and the like.

For anodes, the fuel near the inlet is predominantly a hydrocarbon, butalong the flow path, reforming or partial oxidation processes canproduce carbon monoxide and hydrogen, which may be among the dominantgases further downstream, especially under conditions of high fuelutilization. Although catalysts that can reform, oxidize, or partiallyoxidize a hydrocarbon fuel can typically oxidize carbon monoxide,hydrogen, and/or other partial oxidation products, they are notoptimized for these gases. In contrast, embodiments of the presentinvention choose compositional and/or structural gradients of the anodematerial so as to increase catalytic activity of the anode dependingupon where in the flow path the anode or discrete area of the anode ispositioned.

With regard to cathodes, the air near the inlet has not yet beendepleted of oxidants (e.g. is rich with oxygen), whereas furtherdownstream, the air becomes partially depleted or substantially depletedof oxidants. Embodiments of the present invention choose compositionaland/or structural gradients of the cathode material so as to increasecatalytic activity of the cathode depending upon where in the flow paththe cathode or discrete area of the cathode is positioned.

Referring now to FIG. 1, an embodiment of the fuel cell of the presentinvention is designated generally as 10. Fuel cell 10 includes a flowpassage 24 having a gas stream flowing therethrough in the direction ofarrow A. Fuel cell 10 further includes at least one electrode 16, 18operatively disposed in the flow passage 24. The electrode may be ananode 16 and/or a cathode 18. The electrode(s) are part of a fuel cellassembly 12, which includes an electrolyte 14, an anode 16 disposed onone side of the electrolyte 14, and a cathode 18 disposed on the same orthe other side of the electrolyte 14. It is generally desirable for fuelcell 10 to include a plurality of fuel cell assemblies 12.

It is to be understood that the fuel cell 10 may be one of solid oxidefuel cells, proton conducting ceramic fuel cells, alkaline fuel cells,Polymer Electrolyte Membrane (PEM) fuel cells, molten carbonate fuelcells, solid acid fuel cells, and Direct Methanol PEM fuel cells. In anembodiment of the present invention, fuel cell 10 is a solid oxide fuelcell.

In the embodiment of FIG. 1, the fuel cell 10 is an example of a singlechamber fuel cell. In single chamber fuel cells, it may be desirable tospace apart adjacent fuel cell assemblies 12 so as to promote gastransport to more of the catalytically active surfaces of the anode 16and cathode 18. The stacking order of these cells may be anode 16/gaschannel 46/anode 16; or anode 16/gas channel 46/cathode 18.Alternatively and optionally, the cell 10 could be stacked anode16/electrolyte 14/cathode 18/electrolyte 14/anode 16/electrolyte 14,etc. without gas channels 46 between adjacent fuel cell assemblies 12.

The electrode 16, 18 has at least one discrete, catalytically activearea, the composition and/or the structure of which is predeterminedbased upon an expected composition of the gas stream to which thediscrete area is exposed. If the electrode is anode 16, the discretecatalytically active areas are designated as 16′, 16″ and 16′″. Althoughthree discrete areas 16′, 16″ and 16′″ are shown, it is to be understoodthat anode 16 may include any number of discrete catalytically activeareas as desired, or may continuously vary along the indicateddirection. If the electrode is cathode 18, the discrete catalyticallyactive areas are designated as 18′, 18″ and 18′″. As with anode 16, itis to be understood that although three discrete areas 18′, 18″ and 18′″are shown, cathode 18 may include any number of discrete catalyticallyactive areas as desired, or may continuously vary along the indicateddirection.

Fuel cell 10 further includes an inlet 20 adjacent an entrance to flowpassage 24, wherein the electrode 16, 18 has an inlet end region 26proximate the inlet 20, and wherein the discrete area 16′, 18′ islocated at the inlet end region 26. It is to be understood that inlet 20may be an inlet for fuel, oxidants, or both fuel and oxidants. If theelectrode is an anode 16, the expected composition of the gas stream atthe inlet end region 26 is generally substantially unreformedhydrocarbon fuel. As such, according to an embodiment of the presentinvention, the composition and/or the structure of discrete area 16′ isoptimized for substantially unreformed hydrocarbon fuel.

If the electrode is a cathode 18, the expected composition of the gasstream at inlet end region 26 is a gas stream substantially undepletedof oxidants. As such, according to an embodiment of the presentinvention, the composition and/or the structure of discrete area 18′ isoptimized for a gas stream substantially undepleted of oxidants.

Fuel cell 10 further includes an outlet 22 adjacent an exit from flowpassage 24. The electrodes 16, 18 have an outlet end region 28 proximatethe outlet 22. Discrete areas 16′″, 18′″ are located at the outlet endregion 28.

If the electrode is an anode 16, the expected composition of the gasstream at outlet end region 28 is at least one of substantially reformedor partially reformed hydrocarbon fuel, byproducts thereof, and mixturesthereof. As such, according to an embodiment of the present invention,the composition and/or the structure of discrete area 16′″ is optimizedfor at least one of substantially reformed or partially reformedhydrocarbon fuel, byproducts thereof, and mixtures thereof.

If the electrode is a cathode 18, the expected composition of the gasstream at outlet end region 28 is a gas stream substantially depleted ofoxidants. As such, according to an embodiment of the present invention,the composition and/or structure of discrete area 18′″ is optimized fora gas stream substantially depleted of oxidants.

Flow passage 24 has a midpoint 30, and the electrode 16, 18 has amidpoint region 32 proximate midpoint 30. Discrete area 16″, 18″ islocated at the midpoint region 32.

If the electrode is an anode 16, the expected composition of the gasstream at midpoint region 32 is at least one of substantially unreformedor partially reformed hydrocarbon fuel, byproducts thereof, and mixturesthereof. As such, according to an embodiment of the present invention,the composition and/or structure of discrete area 16″ is optimized forat least one of substantially unreformed or partially reformedhydrocarbon fuel, byproducts thereof, and mixtures thereof.

If the electrode is a cathode 18, the expected composition of the gasstream at midpoint region 32 is a gas stream partially depleted ofoxidants. As such, according to an embodiment of the present invention,the composition and/or structure of discrete area 18″ is optimized for agas stream partially depleted of oxidants.

An electronic device according to the present invention includes anelectrical load L, and fuel cell 10 connected to the load L. Anembodiment of a method of using fuel cell 10 includes the step ofoperatively connecting the fuel cell 10 to electrical load L and/or toan electrical storage device S. The electrical load L may include manydevices, including, but not limited to any or all of computers, portableelectronic appliances (e.g. portable digital assistants (PDAs), portablepower tools, etc.), and communication devices, portable or otherwise,both consumer and military. The electrical storage device S may include,as non-limitative examples, any or all of capacitors, batteries, andpower conditioning devices. Some exemplary power conditioning devicesinclude uninterruptible power supplies, DC/AC converters, DC voltageconverters, voltage regulators, current limiters, etc.

It is also contemplated that the fuel cell 10 of the present inventionmay, in some instances, be suitable for use in the transportationindustry, e.g. to power automobiles, and in the utilities industry, e.g.within power plants.

Alternate embodiments of single chamber fuel cells are shown in FIGS. 1Aand 1B.

Referring now to FIG. 1C, an embodiment of a dual chamber fuel cell isshown, whereby air (as a source of oxidant) is fed to the cathode 18side, and fuel (as a source of reactant) is fed to the anode 16 side. Anoptional additional air inlet 42 is shown in phantom downstream frominlet 20; and an optional additional fuel inlet 44 is shown in phantomdownstream from inlet 20. It is to be understood that, although only oneadditional air/fuel inlet 42, 44 is shown, there may be any number ofinlets 42, 44 as desired. Further, there may be additional air inlet(s)42 with or without additional fuel inlet(s) 44, and vice versa. It is tobe further understood that a manifold and/or similar apparatus,operatively and fluidly connected to the anode 16 side of flow passage24 and/or to the cathode 18 side of flow passage 24, may be provided foradding oxidants and/or fuel in at least one area downstream from inlet20. This may aid in ensuring efficient reactions downstream from theinlet 20, given the composition of the air/fuel and the particularelectrode material at a given location. By adding extra air (i.e.oxygen) at different locations on the cathode 18 side, downstream fromthe inlet 20, partial and total oxidation of the fuel can be controlled.This may reduce the temperature gradient, increase the fuel utilizationand improve the performance of the fuel cell 10. Coking may becontrolled by reducing the concentration of the fuel at specificlocations on the anode 16, and furthermore, may reduce the temperaturegradient on the anode 16. Coking may be defined as the conversion ofsmall chain hydrocarbons to an inactive layer of carbon compounds thatmodify the catalyst in such a way as to reduce performance.

Referring now to FIG. 2, fuel cell 10 is a single chamber fuel cell(FIGS. 3-9 may relate to either single or dual chamber fuel cells) andmay further optionally include additional inlets and/or a manifold 34(shown schematically in FIG. 2) and/or similar apparatus, operativelyand fluidly connected to flow passage 24, for adding oxidants, fueland/or a fuel/air mixture in at least one area downstream from inlet 20.This may aid in ensuring efficient reactions downstream from the inlet20, given the composition of the fuel and the particular electrodematerial at a given location. By adding extra air (i.e. oxygen) atdifferent locations downstream from the inlet 20, partial and totaloxidation of the fuel can be controlled. This may reduce the temperaturegradient, increase the fuel utilization and improve the performance ofthe fuel cell 10. By adding extra fuel and/or an air/fuel mixture atdifferent locations downstream from the inlet 20, the dilution effect(due to hydrocarbon fuels' production of reaction product(s)) may becontrolled.

Along the fuel path, the fuel may react to form water, carbon dioxide,carbon monoxide and H₂. Exhaust will result in a dilution effect, andair adds N₂ as well. Conventional fuel cells have a single ratio of fuelto air along the reaction path, whereas in embodiments of the presentinvention, the ratio of fuel to air is varied along the reaction path.In addition, a single chamber design of a fuel cell 10 according to anembodiment of the present invention may have a compositional gradient ofboth the anode 16/cathode 18 material, and the gas phase reactants(adding air downstream to control the composition of the gas).

Referring now to FIG. 3, in a non-limitative embodiment of the anode 16of the present invention, discrete area 16′ has as a main componentthereof. LaCr(Ni)O₃, the composition of discrete area 16″ has as a maincomponent thereof La(Sr)CrO₃, and the composition of discrete area 16′″has as a main component thereof La(Sr)Cr(Mn)O₃. This is an example of acompositional gradient of the anode material 16 which allows for morecomplete utilization of the fuel. Higher performance may be obtained bycontrolling the catalyst and the resulting gas composition. The LaCrO₃perovskite system of FIG. 3 is one non-limitative example of optimizingcatalytic activity of an anode 16 according to embodiments of thepresent invention.

Doping the A and B sites of the perovskite lattice may significantlyalter the observed catalytic activity and selectivity. The nomenclatureis A(C)B(D)O₃, where A and B are the specific sites in the perovskitestructure, and C and D are the dopants on the sites.

It has been observed that LaCr(Ni)O₃ is good for methane conversion andreforming reactions, La(Sr)CrO₃ is good for carbon monoxide oxidation,and La(Sr)Cr(Mn)O₃ is good for hydrogen oxidation.

It is to be understood that material systems other than those describedherein may be used as well, depending on the desired characteristics andthe fuels to be used.

Referring now to FIG. 4, in an alternate non-limitative embodiment ofthe anode 16 of the present invention, the composition of discrete area16′″ includes a first amount of nickel in an anode material (forexample, a samaria doped ceria (SDC)), the composition of discrete area16″ includes a second amount of nickel, which is less than the firstamount of nickel, and the composition of discrete area 16′ includes athird amount of nickel, which is less than the second amount of nickel.

Nickel assists in reaction of hydrocarbons. However, nickel (and/orother metals which assist in reaction of hydrocarbons) may causeundesirable temperature gradients which may lead to cracking of the fuelcell 10. For example, with Ni-SDC, most of the reaction occurs proximatethe fuel inlet, and this causes a temperature gradient (the fuel cellfilms/film stacks are hotter at the inlet 20 than at the outlet 22). Anembodiment of the present invention as shown in FIG. 4 provides a moreconstant temperature across the anode 16/fuel cell 10 by lowering theamount of nickel at the fuel inlet 20. This compositional gradient ofthe anode material 16 allows for more even heating of the anode 16during exothermic reactions so that the inlet end region 26 of the anode16 does not become overheated. This may reduce stress related todifferent thermal expansion at different regions of the fuel cell 10.

With better control over the heat given off by the exothermic reactions,other components of the fuel cell 10 may advantageously be optimized forthe lower temperature operation.

In addition or alternatively to selectively varying the ratio of nickeland/or other metals, it is contemplated as being within the purview ofthe present invention to vary the ceramic ratio, vary doping, etc.

Referring now to FIG. 5, a non-limitative embodiment of an anode 16structural gradient is shown. The structure of discrete area 16′″includes pores 36, the structure of discrete area 16″ includes pores 36smaller than the pores 36 in discrete area 16′″, and the structure ofdiscrete area 16′ includes pores 36 smaller than the pores 36 indiscrete area 16″. Structural gradients, in porosity, as well as threephase boundary length, may be controlled at different regions of theanode 16 of embodiments of the present invention. More porous anodes 16may be used at regions with higher exhaust compositions to reducediffusion limitations in the transport of reactive species to theelectrocatalytically active areas; for example, larger pore sizes at theportion 16′″ of the anode 16 reduce diffusional losses related to thetransport of fuel (with a high concentration of CO₂ and H₂O present) tothe three phase boundary in the anode 16. It is to be understood thatFIG. 5 is a very simplified representation. For example, the structuralgradient may not simply be smaller to bigger pores 36, it may be anode16 with different pore size distribution(s), e.g. dual distributionwhich may be a combination of large, transport pores for fasterdiffusion and nanopores with a higher concentration of catalyticcenters.

Referring now to FIG. 6, a non-limitative embodiment of a cathode 18compositional gradient is shown. It is to be understood that themain/base material for cathode 18 may be any suitable material, forexample, it may be chosen from examples of cathode materials listedbelow. In an embodiment, an example of a suitable main material forcathode 18 is Sm(Sr)CoO₃ (SSCO).

The composition of discrete area 18′″ includes a first amount ofmaterial catalytically more active (than the main/base cathode 18material) for the electrochemical reduction of molecular oxygen. Themore catalytically active material may aid the reduction of oxygen indepleted atmospheres. It is to be understood that this morecatalytically active material may be any suitable material. In anembodiment, this more catalytically active material is at least one ofplatinum, ruthenium, rhodium, silver, mixtures thereof, and the like.

The composition of discrete area 18″ includes a second amount of themore catalytically active material which is less than the first amountof the more catalytically active material, and further includes a firstamount of material catalytically less active than the main/base cathode18 material. It is to be understood that this catalytically less activematerial may be any suitable material. In an embodiment, thiscatalytically less active material is at least one of iron, manganese,mixtures thereof, and the like.

The composition of discrete area 18′ includes a second amount of thecatalytically less active material, which is more than the first amountof catalytically less active material. Without being bound to anytheory, it is believed that the addition of the less catalyticallyactive material will typically result in less active materials than thepure main material (e.g. SSCO), but may better match the thermalexpansion properties of the other components in the fuel cell 10. Sincethe inlet usually runs hotter, this may help reduce delamination orother stress in the cell.

Referring now to FIG. 7, an embodiment of a cathode 18 structuralgradient according to the present invention is shown. The structure ofdiscrete area 18′″ includes pores 38; the structure of discrete area 18″includes pores 38, which are smaller than the pores 38 in discrete area18′″; and the structure of discrete area 18′ includes pores 38 smallerthan the pores 38 in discrete area 18″. It is believed that increasingthe size of pores 38 downstream from inlet 20 advantageously allowshigher diffusional mass transport to the active areas in the cathode 18when there is a low(er) concentration of molecular oxygen in the airstream. It is to be understood that FIG. 7 is a very simplifiedrepresentation. For example, the structural gradient may not simply besmaller to bigger pores 38, it may be cathode 18 with different poresize distribution(s), e.g. dual distribution which may be a combinationof large, transport pores for faster diffusion and nanopores with ahigher concentration of catalytic centers.

According to embodiments of the present invention, the compositionaland/or structural gradient for the electrodes may also be incorporatedinto fuel cells stacks. The composition and/or structure of a specificanode 16/cathode 18 in the stack may be predetermined relative to itsposition along the gas flow path. Referring now to FIGS. 8A and 8B,embodiments of fuel cell stacks 40, 40′ are shown in schematic blockdiagrams. It is to be understood that when anode(s) 16 is shown (as inFIG. 8A), it is (although not shown) associated with an adjacentelectrolyte 14 and cathode 18 to form a fuel cell assembly 12. Likewise,when cathode(s) 18 is shown (as in FIG. 8B), it is to be understood thatit is associated with an adjacent electrolyte 14 and anode 16 to form afuel cell assembly 12.

Fuel cell stack 40, 40′ includes an inlet 20, an outlet 22, and a flowpassage 24 disposed between inlet 20 and outlet 22 and having a gasstream flowing therethrough. A plurality of electrodes 16, 18 isoperatively positioned within the flow passage 24 from proximate inlet20 to proximate outlet 22 and positions therebetween. According to anembodiment of the present invention, the structure and/or thecomposition of each of the plurality of electrodes 16, 18 ispredetermined based upon an expected composition of the gas stream at anarea of the fuel cell stack 40, 40′ in which the electrode ispositioned.

In FIG. 8A, each of the plurality of electrodes is an anode 16; and inFIG. 8B, each of the plurality of electrodes is a cathode 18. Althoughthree anodes/cathodes A, B, C are shown, it is to be understood thatfuel cell stacks 40, 40′ may include any number of individual anodes16/cathodes 18 as desired and/or necessitated by a particular end use.As non-limitative examples, anode/cathode A, anode/cathode B andanode/cathode C are as follows.

According to an embodiment of the present invention, the compositionand/or the structure of anode A is optimized for substantiallyunreformed hydrocarbon fuel. According to an embodiment of the presentinvention, the composition and/or the structure of cathode A isoptimized for a gas stream substantially undepleted of oxidants.

According to an embodiment of the present invention, the compositionand/or the structure of anode C is optimized for at least one ofsubstantially reformed or partially reformed hydrocarbon fuel,byproducts thereof, and mixtures thereof. According to an embodiment ofthe present invention, the composition and/or structure of cathode C isoptimized for a gas stream substantially depleted of oxidants.

According to an embodiment of the present invention, the compositionand/or structure of anode B is optimized for at least one ofsubstantially unreformed or partially reformed hydrocarbon fuel,byproducts thereof, and mixtures thereof. According to an embodiment ofthe present invention, the composition and/or structure of cathode B isoptimized for a gas stream partially depleted of oxidants.

It is to be understood that the compositions and/or structures ofanode/cathode A, B, C may be chosen from any examples given above. As anon-limitative example, the composition of anode A may have as a maincomponent thereof. LaCr(Ni)O₃, which is an example given above fordiscrete area 16′. The composition of cathode A may be SSCO with anamount of iron (which is an example given above for discrete area 18′)larger than an amount of iron in cathode B. Similarly, any of theexamples, and/or combinations thereof, of compositions and/or structuresgiven for discrete areas 16′/18′, 16″/18″ and 16′″, 18′″ may be used foranode/cathode A, anode/cathode B and anode/cathode C, respectively.

Further, it is to be understood that in addition to the compositionand/or structure of each anode/cathode A, B, C being individuallyuniform (as described immediately hereinabove), any, some or all of theindividual anodes/cathodes A, B, C may have compositional and/orstructural gradients thereon (e.g. anode A may include any, all orfurther discrete areas 16′, 16″ and 16′″).

Referring now to FIG. 9, one method of growing anode materials withcompositional gradients is shown. An embodiment of a method for making afuel cell anode 16 includes the step of depositing a first film on afirst end region 16′ of a substrate, wherein the first film ispreferentially catalytically active toward substantially unreformedhydrocarbon fuel. The method may further include the step of depositinga second film on a second end region 16′″ of the substrate opposed tothe first end region 16′, wherein the second film is preferentiallycatalytically active toward at least one of substantially reformed orpartially reformed hydrocarbon fuel, byproducts thereof, and mixturesthereof.

The method may further optionally include the step of depositing anintermediate film on a region 16″ of the substrate intermediate thefirst end region 16′ and the second end region 16′″, wherein theintermediate film is preferentially catalytically active toward at leastone of substantially unreformed or partially reformed hydrocarbon fuel,byproducts thereof, and mixtures thereof.

It is to be understood that the dashed lines between discrete areas 16′,16″ and 16′″ in FIG. 9 represent that this embodiment of the method ofthe present invention generally results in a gradient distribution ofthe deposited materials, i.e. a continuous, inhomogeneous distribution.It is to be further understood that, in any of the embodiments discussedherein, discrete areas 16′, 16″, 16′″, 18′, 18″, 18′″ may also have agradient distribution between adjacent areas, i.e. a continuous,inhomogeneous distribution.

In an embodiment of the method of the present invention, each of thefirst, intermediate, and second films has as a main component thereof anickel-samaria doped ceria cermet, and the method further includes thestep of biasing inclusion of nickel toward the second film.

In an alternate embodiment of the method of the present invention, thefirst film has as a main component thereof LaCr(Ni)O₃, the intermediatefilm has as a main component thereof La(Sr)CrO₃, and the second film hasas a main component thereof La(Sr)Cr(Mn)O₃.

An embodiment of a method of the present invention for making a fuelcell cathode 18 includes the step of depositing a first film on a firstend region 18′ of a substrate, wherein the first film is preferentiallycatalytically active toward a gas stream substantially undepleted ofoxidants. The method may further include the step of depositing a secondfilm on a second end region 18′″ of the substrate opposed to the firstend region 18′, wherein the second film is preferentially catalyticallyactive toward a gas stream substantially depleted of oxidants.

The method may further optionally include the step of depositing anintermediate film on a region 18″ of the substrate intermediate thefirst end region 18′ and the second end region 18′″, wherein theintermediate film is preferentially catalytically active toward a gasstream partially depleted of oxidants.

In an embodiment of the method of the present invention, each of theintermediate and second films has therein an amount of a materialcatalytically more active (than a main/base cathode 18material/substrate) for the electrochemical reduction of molecularoxygen (some suitable, non-limitative examples of the more catalyticallyactive material are as described hereinabove). The method furtherincludes the step of biasing inclusion of the more catalytically activematerial (e.g. platinum) toward the second film.

In an embodiment of the method of the present invention, each of thefirst and intermediate films has therein an amount of a materialcatalytically less active than a main/base cathode 18 material/substrate(some suitable, non-limitative examples of the less catalytically activematerial are as described hereinabove). The method further includes thestep of biasing inclusion of the less catalytically active material(e.g. iron) toward the first film.

Without being bound to any theory, it is believed that embodiments ofthe method of the present invention may result in changes inmorphology/structure, as well as in composition. Angular deposition mayresult in porous materials depending on many factors, two of whichfactors are adatom mobility (material and temperature dependent, alsodependent on other parameters which may affect the energy of the adatomwhen it reaches the surface of the substrate: process pressure, power,substrate bias, target-to-substrate distance, and the like), andself-shadowing due to nucleation and growth of islands (due to the lowdeposition angles).

As such, it is to be understood that the first, intermediate and secondanode films may also include pores 36 (such as pores 36 in discreteareas 16′, 16″ and 16′″, respectively, as shown in FIG. 5), and/or otherchanges in morphology. It is to be further understood that the first,intermediate and second cathode films may also include pores 38 (such aspores 38 in discrete areas 18′, 18″ and 18′″, respectively, as shown inFIG. 7), and/or other changes in morphology.

Further, it is to be understood that several different methods may beused to make the compositional gradients of embodiments of the presentinvention, including but not limited to sputter deposition,impregnation, dip coating or other means, and the like. Further methodsinclude, but are not limited to asymmetric screen printing and/orasymmetric tape casting, both generally with dopant or pore formerdelivery from one side, colloidal spray deposition, and the like.Substantially all deposition methods are contemplated as being withinthe purview of the present invention, provided that there is someasymmetry (i.e., two-sources or more with different compositions whereinthe sources will not provide a homogenous distribution on the substrate,e.g. one source biased to one end and the other source biased to theother end).

It is to be understood that the electrolyte 14 may be formed from anysuitable material. In an embodiment of the present invention,electrolyte 14 is at least one of oxygen ion conducting membranes,proton conductors, carbonate (CO₃ ²⁻) conductors, OH⁻ conductors, andmixtures thereof.

In an alternate embodiment, electrolyte 14 is at least one of cubicfluorite structures, doped cubic fluorites, proton-exchange polymers,proton-exchange ceramics, and mixtures thereof. In a further alternateembodiment, electrolyte 14 is at least one of yttria-stabilizedzirconia, samarium doped-ceria, gadolinium doped-ceria,La_(a)Sr_(b)Ga_(c)Mg_(d)O_(3-δ), and mixtures thereof.

It is to be understood that the anode 16 and cathode 18 may be formedfrom any suitable material, as desired and/or necessitated by aparticular end use. In an embodiment, each of the anode 16 and cathode18 is at least one of metals, ceramics and cermets.

In an embodiment of the present invention, some non-limitative examplesof metals which may be suitable for the anode 16 include at least one ofnickel, platinum, palladium, and mixtures thereof. Some non-limitativeexamples of ceramics which may be suitable for the anode 16 include atleast one of Ce_(x)Sm_(y)O_(2-δ), Ce_(x)Gd_(y)O_(2-δ),La_(x)Sr_(y)Cr_(z)O_(3-δ), and mixtures thereof. Some non-limitativeexamples of cermets which may be suitable for the anode 16 include atleast one of Ni-YSZ, Cu-YSZ, Ni-SDC, Ni-GDC, Cu-SDC, Cu-GDC, andmixtures thereof.

In an embodiment of the present invention, some non-limitative examplesof metals which may be suitable for the cathode 18 include at least oneof silver, platinum, ruthenium, rhodium, and mixtures thereof. Somenon-limitative examples of ceramics which may be suitable for thecathode 18 include at least one of Sm_(x)Sr_(y)CoO_(3-δ),Ba_(x)La_(y)CoO_(3-δ), Gd_(x)Sr_(y)CoO_(3-δ), and mixture thereof.

In any of the embodiments described herein, the gas to which fuel cell10 is exposed includes reactants and/or oxidants and/or mixturesthereof. In an embodiment, the reactants are fuels, and the oxidants areone of oxygen, air, and mixtures thereof.

It is to be understood that any suitable fuel/reactant may be used withthe fuel cell 10 of the present invention. In an embodiment, thefuel/reactant is selected from at least one of hydrogen, methane,ethane, propane, butane, pentane, methanol, ethanol, higher straightchain or mixed hydrocarbons, for example, natural gas or gasoline (lowsulfur hydrocarbons may be desirable, e.g. low sulfur gasoline, lowsulfur kerosene, low sulfur diesel), and mixtures thereof. In analternate embodiment, the fuel/reactant is selected from the groupconsisting of butane, propane, methane, pentane, and mixtures thereof.Suitable fuels may be chosen for their suitability for internal and/ordirect reformation, suitable vapor pressure within the operatingtemperature range of interest, and like parameters.

It is to be understood that the “expected compositions” of gas describedherein are non-limitative, and for illustrative purposes. As such, it isto be understood that the discrete areas 16′/18′, 16″/18″, 16′″/18′″and/or individual anodes/cathodes A, B, C should be optimized forwhatever fuel is chosen, and its reaction and consequent byproductsalong the fuel flow path.

In an embodiment of the present invention, the fuel cell 10 is a singlechamber fuel cell (FIGS. 1, 1A and 1B). FIG. 2 is an example of an anodefuel/air mixture gradient for a single chamber fuel cell. In embodimentsof single chamber fuel cells, the gas is a mixture of reactants andoxidants.

It is to be understood that it is not necessary for good performance ofthe fuel cell 10 to have leak tight separation between air, fuel andexhaust in embodiments of the present invention relating tosingle-chamber fuel cells. When mixing fuel, air and/or exhaust, it maybe desirable to keep the dimensions in the fuel cell stack below thecritical length required for propagation of a flame; e.g. forhydrocarbons, a flame generally needs to be at least about 1-3 mm insize to exist at room temperature. Optionally or additionally, it may bedesirable to adjust the air-fuel mixture so as to run with excess (abovethe upper flammability limit) fuel (for example, the upper flammabilitylimit for propane is 9.6%); and then to add more air when the oxygen isconsumed later in the stack. It may be desirable to add air at severallocations in the stack. Alternately to running with excess fuel, it maybe desirable to adjust the air-fuel mixture so as to run with excess(below the lower flammability limit) air (for example, the lowerflammability limit for propane is 2.2%); and then to add more fuel whenthe fuel is consumed later in the stack. It may be desirable to add fuelat several locations in the stack. It is believed apparent that amixture of multiple flammable gases will have a different flammabilitylimit than the flammability limit of the gases individually. Thus, iffor example, carbon monoxide (as a reaction product) is combined withpropane (as a fuel) later in the cell, the lower flammability limit ofthe mixture is 3.3%, while the upper limit is 10.9% according to LeChâtelier's Principle.

In an alternate embodiment of the present invention, the fuel cell 10 isa dual chamber fuel cell (FIG. 1C). In embodiments of dual chamber fuelcells, the gas is one of reactants and oxidants. Oxidants are carried tothe cathode 18 of each of the fuel cell assemblies 12, and reactants arecarried to the anode 16 of each of the fuel cell assemblies.

It is to be understood that the gas flow may be in any suitabledirection as desired and/or necessitated by a particular end use. Forexample, the gas flow direction may be a direction reverse of thatindicated by arrow A (FIG. 1), if desired. If such gas flow direction isreversed, it is to be further understood that inlet 20 and outlet 22would also be the reverse of those shown in the figures, and discreteareas 16′, 18′ and 16′″, 18′″ would be the reverse of those shown in thefigures.

It is to be understood that the anode 16 and/or cathode 18 are to beoptimized according to an expected composition of the gas to which it16, 18 is exposed. It is to be further understood that many embodimentsof the anodes 16/cathodes 18 are contemplated as being within thepurview of the present invention. For example, each of anode 16 and/orcathode 18 may include any of the appropriate structures and/orcompositions for discrete areas 16′/18′, 16″/18″ and 16′/18′″ (as wellas other appropriate structures/compositions), in any combinationthereof. As one non-limitative example, discrete area 16′″ of anode 16may be formed from Ni_(y):Ce_(1-x)Sm_(x)O₂, and also may include largepores 36 as shown in FIG. 5.

The gas phase and/or compositional and/or structural gradients of theanodes 16/cathodes 18 of embodiments of the present invention allow forbetter fuel utilization, better thermal stability of the fuel cell10/stack 40, 40′, and enhanced performance.

While several embodiments of the invention have been described indetail, it will be apparent to those skilled in the art that thedisclosed embodiments may be modified. Therefore, the foregoingdescription is to be considered exemplary rather than limiting, and thetrue scope of the invention is that defined in the following claims.

1. A fuel cell, comprising: a flow passage having a gas stream flowingtherethrough; and at least one anode operatively disposed in the flowpassage, and having a first end region that contains LaCr(Ni)O₃ and ispreferentially catalytically active toward substantially unreformedhydrocarbon fuel, and a second end region opposed to the first endregion, wherein the second end region contains La(Sr)Cr(Mn)O₃ and ispreferentially catalytically active toward at least one of substantiallyreformed or partially reformed hydrocarbon fuel, byproducts thereof, andmixtures thereof.
 2. The fuel cell as defined in claim 1, furthercomprising an inlet adjacent an entrance to the flow passage, whereinthe at least one anode has an inlet end region proximate the inlet, andwherein the first end region is located at the inlet end region.
 3. Thefuel cell as defined in claim 1, further comprising an outlet adjacentan exit from the flow passage, wherein the at least one anode has anoutlet end region proximate the outlet, and wherein the second endregion is located at the outlet end region.
 4. The fuel cell as definedin claim wherein the flow passage has a midpoint, wherein the at leastone anode has a midpoint region proximate the midpoint, and wherein athird region containing La(Sr)CrO3 is located at the midpoint region. 5.A fuel cell, comprising: a flow passage having a gas stream flowingtherethrough; and an inlet at one end of the flow passage; at least oneanode operatively disposed in the flow passage, and having a first endregion that contains LaCr(Ni)O₃ and is preferentially catalyticallyactive toward substantially unreformed hydrocarbon fuel, and a secondend region opposed to the first end region, wherein the second endregion contains La(Sr)Cr(Mn)O₃ and is preferentially catalyticallyactive toward at least one of substantially reformed or partiallyreformed hydrocarbon fuel, byproducts thereof, and mixtures thereof; anda manifold, operatively and fluidly connected to the flow passage, foradding at least one of reactants or oxidants in at least one areadownstream from the inlet.
 6. An electronic device, comprising: a load;and the fuel cell of claim 1 connected to the load.
 7. A fuel cell,comprising: a flow passage having a gas stream flowing therethrough, theflow passage having a midpoint; an inlet adjacent an entrance to theflow passage; an outlet adjacent an exit from the flow passage; and atleast one anode operatively disposed in the flow passage, and having afirst, second and third discrete, catalytically active area, the firstdiscrete catalytically active area containing LaCr(Ni)O₃, the seconddiscrete catalytically active area containing La(Sr)CrO3, and the thirdcatalytically active area containing La(Sr)Cr(Mn)O₃; wherein the atleast one electrode has an inlet end region proximate the inlet, anoutlet end region proximate the outlet, and a midpoint region proximatethe midpoint, and wherein the first discrete area is located at theinlet end region, the second discrete area is located at the midpointregion, and the third discrete area is located at the outlet end region.8. The fuel cell as defined in claim 7 wherein the first expectedcomposition of the gas stream comprises substantially unreformedhydrocarbon fuel; wherein the second expected composition of the gasstream comprises at least one of substantially unreformed or partiallyreformed hydrocarbon fuel, byproducts thereof, and mixtures thereof; andwherein the third expected composition of the gas stream comprises atleast one of substantially reformed or partially reformed hydrocarbonfuel, byproducts thereof, and mixtures thereof.
 9. The fuel cell asdefined in claim 8 wherein the structure of the third discrete areacomprises pores, wherein the structure of the second discrete areacomprises pores smaller than the third discrete area pores, and whereinthe structure of the first discrete area comprises pores smaller thanthe second discrete area pores.
 10. The fuel cell as defined in claim 8wherein the structure of at least one of the first, second and thirddiscrete areas comprises a varied pore size distribution.
 11. The fuelcell as defined in claim 10 wherein the varied pore size distributionincludes a dual distribution comprising a combination of large,transport pores and nanopores.
 12. The fuel cell as defined in claim 7,further comprising a manifold, operatively and fluidly connected to theflow passage, for adding at least one of reactants or oxidants in atleast one area downstream from the inlet.
 13. A single chamber fuel cellstack, comprising: an inlet; an outlet; a flow passage disposed betweenthe inlet and the outlet, and having a gas stream flowing therethrough;and a plurality of electrodes each having at least two discrete,catalytically active areas, each of the discrete areas having astructure and a composition, and operatively positioned within the flowpassage from proximate the inlet to proximate the outlet and positionstherebetween, wherein at least one of the structure or composition ofone of the discrete areas is different from at least one of thecomposition or structure of an other of the discrete areas of each ofthe plurality of electrodes, the at least one of the composition orstructure of each of the at least two discrete areas being predeterminedbased upon an expected composition of the gas stream at an area of thefuel cell stack in which the electrode is positioned.
 14. The fuel cellstack as defined in claim 13 wherein the plurality of electrodesincludes anodes and cathodes.
 15. The fuel cell of claim 1, furthercomprising: an inlet at one end of the flow passage; and means,operatively and fluidly connected to the flow passage, for adding atleast one of reactants or oxidants in at least one area downstream fromthe inlet, wherein the adding means substantially maintains a uniformmaximum catalytic activity over the surface of the at least one anode.16. The fuel cell as defined in claim 1 wherein a structure of one ofthe first and second end regions includes pores, and wherein a structureof an other of the second and first end regions includes pores having asize different than a size of the pores of the one of the first andsecond end regions.
 17. A fuel cell, comprising: a flow passage having agas stream flowing therethrough; and at least one cathode operativelydisposed in the flow passage, and having a first end regionpreferentially catalytically active toward a gas stream substantiallyundepleted of oxidants, and a second end region opposed to the first endregion, wherein the second end region is preferentially catalyticallyactive toward a gas stream substantially depleted of oxidants, andwherein the first end region contains an increased iron content and adecreased platinum content compared to the second end region.
 18. Thefuel cell of claim 17, further comprising: an inlet at one end of theflow passage; and means, operatively and fluidly connected to the flawpassage, for adding at least one of reactants and oxidants in at leastone area downstream from the inlet.